57 



Figure 37 : Veliconcha of Crassostrea gigas. 

 A — external view; B — left valve. 



15(14). Anterior margin acute. Ventral margin appreciably rounded not par- 

 allel to hinge line. Concentric line of lesser relief indistinct. 



16(17). Provinculum consisting of a continuous row of denticles 



Mytilidae, Figures 44, 52, 53, 54. 



17(16). Denticles absent in central part of provinculum Arcidae. 



18(11). Margins of shell not rounded. 



19 (22). Posterior margin of shell straight; anterior margin acute and stretched. 



20(21). Posterior margin appreciably slopes towards ventral margin. Um- 

 bones low and flat. Dark pigmentation present along mantle margin 

 and shoulder Myidae, Figures 45, 62. 



2 1 (20). Posterior margin slopes slightly and almost perpendicular to ventral 

 margin. Umbones are high conical. Pigmentation absent along the 

 shoulders and mantle margin Mactridae, Figures 46, 60, 61. 



22(19). Posterior margin of shell rounded; anterior margin not tapering or 

 almost net, and not stretched more than posterior margin. 



23 (26). Shell triangular-oval with triangular apex in umbonal region. Shoul- 

 ders drop steeply toward ventral side. 



24(25). Ventral and lateral sides form regular semicircle. Shoulders of al- 

 most equal or equal length, symmetrical. Pallial line absent. Height 

 of shell equal to its length or slightly less . . . Veneridae, Figure 47. 



25 (24). Ventral and lateral sides semioval. Anterior shoulder longer than 

 posterior and slightly bent. Pallial line present. Height of shell less 

 than its length Hiatellidae, Figures 46, 58. 



